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Stylolites!!! - A diagnostic tool?

Introduction

The chances are that the request "tiles or slabs with stylolites"
from your supplier, will draw a blank stare at best, even if he has
supplied the slabs to you for who-knows-how-long. A benign nod to your
request still does not mean that he is spending much thought on
stylolites. Why should we?

During the last century the use of stone slabs instead of solid
masonry in buildings, facades, claddings has been significantly
increasing, in addition to flooring and paving, presently the most used
product accounting for nearly 40% of the global stone consumption.

In the selection of stone for the application we are helped by
checking the geotechnical features, mainly mechanical and physical
properties and awareness of these is the more important since flooring
and paving, has been much in the limelight lately for several reasons,
besides being a leading product.

Such
attention included: firstly safety issues concerning slip proofing,
resulting after many discussions, in a Slip Resistance Standard EN
14231; secondly the quest for the best way to consolidate with resins
quarry blocks with open veins or cracks, and thirdly how to prevent
opening of sutures, veins and avoid spalling in floor surfaces by
choosing the right rock orientation. The latter depends on the inherent
stone properties, the genesis and tectonic history of the rock mass, to
which little attention is paid by the industry.

Little consideration is given to the internal structure of the
stone. And still less has been written about the influence of
stylolites, a major parameter, as a means to select a suitable stone,
to foresee its effect on applications and their performance thereafter,
not to speak of using stylolites as a diagnostic tool already during
prospecting for stone deposits.

This disregard resulted in defaults, especially in limestones slabs
and tiles with pre-existing discontinuities in the stone, like
stylolites. The same applies to installation defaults, ie proper
attachment and prevention of slabs breaking during processing,
installation, or after placing. Actually a great deal, if not most,
litigations and complaints in the stone sector concern floor coverings
containing problematic stylolites cut with-the bed, in addition to
slipping, tripping and stumbling accidents.

In a sense the industry can be hold partly responsible for this
state of affairs, by the lack of geotechnical attention to stylolites
during prospection and extraction at the quarries. In addition proper
attention to possible defaults provides the quarry owner or stone
supplier with a better understanding of the deposit and ensures the
clients satisfaction.

What
happens after the stone leaves the quarry, during and beyond
processing, receives little dialogue, a situation partly due to limited
outreach by the geo-technologists who in turn have their difficulties,
not surprisingly in making geological gobbledegook palatable to
industry. Mea culpa. The terms used for geological processes lean
heavily on tectonical and petrographical terms which do not evoke the
same connotations to the industry as the more commonly used mechanical
and structural parameters of daily use in industry and construction.
Demystifying the complex terminology used by geoscientists and
harmonization with terms of the industry or interested outsiders, will
be an important contribution. Informal explanations for some terms used
are given in the boxes*

PRACTICAL IMPLICATIONS

Whereas the use of stylolites to diagnose tectonics during for
opening quarries may appear esoteric, the important role of bedding
planes, one of the more obvious genetically features of a deposit or
quarry face is easily explainable. A regular bedding plane facilitates
extraction of blocks. The relation of stylolites to bedding planes is
clear when considering cutting and sawing directions of stone.
Stylolites act as orientation indicators and have a bearing on
performance tests requirements, especially in stone strength testing,
where testing in two directions of the stone to be used is mandatory.
This requirement exists in several other stone standards tests; one
test in the bedding direction and the other at right angles to the
bedding plane.

These properties are known colloquially as "with the bed" or
"on-bed", "against the bed", perpendicular-to-the-bed or edge bedded as
the case may be. Environmental uses or conditions play an important
part. For example, the same stone performing perfectly in cladding
on-bed may have unsightly open seams when used in flooring where water
action, by continuous dampness, ponding, residual water, or toxic
cleaning may slowly dissolve the clayey or marly stylolite infillings.
Surface spalling is another result from using stylolitic limestones cut
on-bed. The interpretation of the stylolite geometry may eventually
help to minimize these effects.

Stylolitic stone varieties

Stylolites are found in many rock types including sandstones, to a
lesser extent igneous rocks, and other deposits metamorphosed to
various degrees. However, it are the limestones and dolomites, used in
building and construction that form the largest group influenced by the
presence of stylolites, especially those stones with a high pure
calcareous content.

Our attention is therefore turned to the stylolitic limestones that
are used for decorative and ornamental uses, and often marketed as
marble.(the term is used in a commercial sense).

Geotechnically
we are not concerned with colour aesthetics, except perhaps where
fading and possibly discolouration may occur. However grouping by shade
is helpful as most users specify in the first place colour rather than
other properties in selection. We will consider some international or
locally popular varieties (mainly from those denominated in
Denomination Criteria Standard EN 12440) containing stylolites. The
terms in brackets give, where typical, the stylolitic pattern/colour
effect of the containing stylolites in the stone varieties listed:
geometrical -single ,net, rich,+ veins ; seam colour - faint, yellow,
greenish, black.

Diagnostic potential of stylolites

Behaviour of stone materials can be traced to conditions during the
rock genesis and to the tectonic features of the rock mass.
Discontinuities (sudden changes of rock features) originate during the
rock genesis and to the successive tectonic events in the quarry area.
The quarry investigations and laboratory measurements have then to be
linked to the limestone applications in building frontages or paving.

The material characterization is made by mechanical tests,
petrophysical measurements and observations of the rock petrography by
optical microscope or electronic instrumentation. The mechanical tests,
particularly direct tensile and bending tests confirm the foresights
made on the slab properties (poor, even nil, mechanical resistance for
some discontinuities, breakage during handling etc.).

Although the overall mechanical properties may be acceptable,
the inherent properties footprinted during genesis or those caused by
historical tectonic discontinuities may at times disappoint during or
after the stone application.
Early investigation of the stone material for potential disorders
prevents default. This requires some knowledge of the stylolites, a
structural examination of the quarry beds to look for discontinuities,
avoiding excessive tectonic stylolites to facilitate commercial block
extraction. The same applies to figuring out cutting orientation to
reduce the 'stratigraphic' stylolites frequency.

Without going in too much details of stylolite geometry, the
interpretation of the various types may provide clues about the genesis
or tectonic history to assist in quality evaluation. Eg presence of
short calcite veins sometimes positioned like ladder like rungs or in
echelon, in association with tectonic stylolites, gives information of
the stress evolution of the slab material, and fracturing proneness
during exposure. The accelerated aging tests, like the Freeze and Thaw
test (EN 12371) confirm this possibility.

Inconclusive discussions on the origin and formation of stylolites
have been, and are still ongoing for over a century now. The subject
though is not quite so academically as it seems. Although an extensive
literature is available on stylolites, little attempt has been made to
apply the data to the dimension stone industry. A lesson may be learned
from oil prospection, where close attention is paid to the origin of
stylolites resulting in important economic implications and where
stylolitic porosity in carbonates is considered a critical factor for
deep hydrocarbon production, and tracing oil traps. Stylolite
development may have resulted in the preservation of early oil
accumulation in place.

Classification

For practical diagnostics, a purely geometric classification of
stylolites is essential, in contrast to the more controversial
genetically classification still awaiting a consensus.

Classification of stylolites is approached in two ways; first by
means of the geometry of the stylolite seam itself and secondly by
means of the congruency of the stylolites in relation to the bedding
plane of the host rock.

1. According to the pure geometry of the stylolites:

Six basic geometric (two-dimensional) types of stylolites have been
differentiated as shown on diagram A. This classification was
originally based on many years of observations in various stratigraphic
horizons in carbonate rocks. These basic configurations are shown
together with a simplified terminology.

This terminology is important for descriptive purposes in the new requirements by EN 12407 "Petrographic Examination".

2. In relation to the bedding plane giving diagnostic indications of
the genesis (with some of the more basic diagnostics described below)
in diagram B: Type 1 represents the most commonly encountered
'horizontal stylolites'. They occur parallel, or nearly parallel to the
bedding of rocks generally not affected by intensive tectonic
structural activity or metamorphism. It is most frequently found in
layered sedimentary rocks, particularly in carbonate rocks.
Type 2 is commonly referred to as 'inclined stylolites' and may be
found in rocks that may have been affected by tectonic structural
activity, as well as in metamorphic and layered igneous rocks.

In type 3, the horizontal stylolites with their higher amplitudes
represent the major seams, which were formed after the inclined
stylolites. The vertical types 4 were formed by pressures at right
angles to the bedding.

Of the two interconnecting types 5 the second with smaller
amplitudes originated under intense tectonic pressure/metamorphism.
Small stylolites are cut or invaded by stylolites with larger amplitude
and thickness, indicating that dissolution processes are not
homogeneous and continuous and that stylolitization may proceed at
different and various times even within the same bed.

In types 6, the inclined stylolite seams have been displaced by near vertical ones.

Conclusions

Besides the importance of stylolites for descriptive purposes for
the compulsory Petrograhic Examination Standard EN 12407, vertical and
inclined stylolites may possibly be used to indicate the quality of the
stone, especially when subject to dynamic loading as on floors or
pavements.

Presently all we know is their presence in areas subjected to
tectonic activities, and observed facts must be accepted as essentially
correct. Research may provide their use in dimension stone as a
pre-test diagnostic tool as valuable as the role of stylolites in
tectonics, for the reconstruction of paleostress directions.

So far the interpretations of stylolites with respect to cutting
directions is an established fact. The application of a
perpendicular-to-the-bed-cut, where appropriate, will prevent many a
failed floor or paving as compared to a with-the-bed-cut.

How are stylolites defined

Generally, stylolites consist of a series of alternating,
penetrating column-like developments, recognizable as irregular planes
of discontinuity between two rock units or masses forming serrated
interfaces; the irregularities are shaped by the columns, 'stylos' in
Greek. These columns which may be conical or cylindrical, make the two
rock units appear to be interlocked or mutually interpenetrating along
a very uneven surface appearing as a suture or parting referred to as a
" stylolite seam". The upper and lower surface (diag. C) of the seam
consist of peaked or stubby broken-off columns, the voids filled with
residual materials e.g. clays, the individual portions of one side of
the seam fitting into the dissolved part of the opposite side.

This surface is most commonly characterized by the concentration of
the relatively insoluble constituents of the two rock units, best seen
at the top of the column (fig A types 2,3). It are the non-carbonate
residues, especially the clay minerals in these seams which may play
havoc with an elegant natural stone floor, and to a lesser extent with
paving.

These relatively insoluble residues within the seams originate by
solution of material at both (sides) of the seam (See box for Genesis
and The Role of Porosity). What do these residues represent? Under the
microscope, bituminous material, clay minerals, silica, dolomite,
sulphides and fluorite are observed within or near the stylolite seams.

Less controversial than the genesis is the division into aggregate
and intergranular types, being rather a definition related to grain
fabrics.

1.- Aggregate stylolites

Any departure of a bedding seam, usually marked by a layer of
insoluble material, could be called a stylolite feature. The height of
the amplitudes (columns) is to be larger than the grain diameter of the
rockpart above and below the stylolite, or the height of the amplitude
is greater than the width of the individual columnar units.

As for the lower limit of the aggregate stylolites, where the
height of the amplitude is smaller than the width of the individual
column, they may be called 'thin residual seams or grooves'. Here it
becomes difficult to draw a line between the pure depositional clay
seams and clay seams or grooves of solution origin.

2.- Intergranular stylolites

One finds frequently stylolite seams with amplitude smaller than the
grain size of the host rock. Such seams may range from macroscopic to
microscopic. In the literature such terms like stylolite seams,
microstylolites, pressure solutes, concave, convex sutured contacts
between two grains, pitted pebbles or oolites are included in the term
intergranular stylolites.

Genesis-when are stylolites formed

The origin of stylolite seams is controversial. The stage of
formation, whether they are formed post-diagenetically
(post-lithification) or during the initial diagenesis
(pre-lithification), vie with the evidence of the physico-chemical
processes involved.

It is easier to visualize that stylolites form during the various
stages of diagenesis i.e. when the rock is created in geological time,
starting at the early burial stage. Then most frequently,
stylolitization is initiated in tectonically and metamorphically
non-affected carbonates. Stylolitization probably ends concordantly
with the complete elimination of pore space by drusy mosaic carbonate.
When cementation approaches its final stages, after a long, drawn out,
process.

Redistribution of materials in some sedimentary bodies before
consolidation, during diagenesis may lead to the formation of
concretions, different cements and some sulphide mineralization (See
Role of Porosity).

Those stylolites formed in indurated lithified rocks, post-
diagenetically, may account for the more steeply crosscutting and
vertical stylolites (Fig B types 5, 6) found predominantly in areas of
tectonic activity or metamorphism where joints, fracture systems and
brecciation are common.

The role of porosity

Unidirectional pressure and the consequent compaction may provide
the pore solution migration necessary for stylolitization. Solution
pressure acts before complete reduction of the pore-space by
cementation, and is indicative of continuity during the diagenesis
stages. Completion early in the burial stage occurs at shallow depth
below the interface between sediment and the overlying water.

Besides the role of porosity in the pressure removal, associated
with stylolite forming during the diagenesis process, it plays an
important role in the creation of cavities, vugs, quartz bearing
concretions etc, often observed in tiles of stylolitic limestone. The
voids are usually filled up during the stone processing before
polishing.

Large crystals or symmetrical crystal aggregates may result in
concretions and differentiations. Similarly the differentiated matter
is deposited in open spaces, pores, fractures, geodic cavities, joints,
fissures and stylolitic seams. Quartz bearing concretions are formed
during a period of silica migration and precipitation of the silica in
cavities, resulting from acid solution of calcareous concretions.

Stylolitization ceases in the final stage of cementation after a
long, drawn out, process throughout the diagenetic history of its host.
During the compaction and cementation of limestone, the contact
boundaries between quartz or silicate grains promote the liquid
capillarity as conducts for diffusion. Solution for stylolitization
continues up to a critical thickness of the residue, depending on the
fabric arrangement of the neighbouring mineral grains, nature of the
residues, permeability, pressure and solution chemistry etc. Stylolite
development continues until porosity is almost eliminated.